A swing valve may be used in a process control system to control a flowing fluid, for example exhaust gas. In one embodiment, such a swing valve may include a body having an outlet and a disc pivotally attached to the body. The disc may be moveable between a closed position and a fully open position. In the closed position, the disc covers the outlet. The flow of gas through the outlet is stopped when the disk is in the closed position. In the fully open position, the disc is pivoted away from the outlet and gas is free to flow through the outlet.
Typically, when the disc begins the transition between the closed position and the fully open position, gas begins to flow through the outlet. When the transition begins and the disc initially cracks open, the flow rate of the gas increases very quickly relative to the angle opening of the disc.
U.S. Pat. No. 7,744,062 discloses an apparatus to vary the effective spring rate of a diaphragm actuator for a control valve. The apparatus includes a first spring operatively coupled to a diaphragm plate and an actuator stem, and a second spring operatively coupled to the actuator stem and configured to be serially operative with the first spring. A movable spring seat is disposed between the first and second springs. While beneficial for controlling actuation, a better system is needed.